Introduction
Initially after the start of the use of XLPE cables, there have been early / premature cable failures which lead the industry wide developments in manufacturing process, materials development and improvements and also material handling. It was identified that the contaminations, protrusion, electrical stress, moisture and/or water were responsible for the degradation of insulation system. In this article we will be focusing mainly on the various options of the manufacturing process, material options and its advantages & disadvantages. Globally nearly 95+% of underground medium voltage (3.3 to 46kV) XLPE Power cables are manufactured with state of art true triple extrusion nitrogen cured technology with the continuous vulcanization process.
Manufacturing Techniques
The XLPE cable manufacturing is highly sophisticated process that must be run with great care to assure that the end product will perform reliably in service for many years. There are many sub processes that must work in tandem with each other. In the manufacturing process, it is very important to note that the conductor, insulation and insulation shield/screen must be cross linked after they are applied to the cable conductor. Cross linking often referred to as curing or vulcanization is a chemical reaction that improves the thermal and mechanical characteristics of these materials providing them with increased strength and stability, particularly at high temperature.
Global Developments in Manufacturing
There are two main types of cross-linking processes that can be used for XLPE insulated power cables:
Moisture Cure (Sioplas) – The chemical (Silane) species are inserted onto the polymer chain, theses species form cross-links when exposed to water. The curing process occurs in the solid phase, after extrusion. The moisture cure approach is almost universally used for making LV cables. In India grafted sioplas technology is predominantly used.
Peroxide cure (Continuous vulcanization) – Thermal decomposition of organic peroxide after extrusion leads to the formation of cross-links between the molten polymer chains in the curing tube. The peroxide cure is the most widely used cross-linking technology globally and used in the manufacturing of the MV (HT), HV & EHV insulated cables.
Initial peroxide cure process used steam curing and due to the increased failures traced back to water treeing, various measures had taken and one of the initiative was the use of dry cure true triple extrusion. Due to increased solubility of water in polyethylene at high temperatures resulting insulation had high moisture content and many voids. This resulted in the development of dry cure technologies amongst which nitrogen is most popular. North America, Europe and developed countries have moved slowly to true triple extrusion dry cured technique since 1980s, which was not sufficient enough to reduce the failure due to water tree and hence most of these geographies have moved to TR-XLPE gradually since 1983. There has been focus towards improvement in cleanliness in compound and clean rooms for material handling during cable manufacturing. The manufacturing improvements are depicted in Figure 1.
Key Comparison of two main Cross-linking processes –
